docs(ngShowHide): make a note of values considered to be falsy

This issue has been a focus of problems for some users and we discussed it on the IRC that it should
be at least documented.

~Amended the style to use bootstrap notes, I think overall it looks better and catches the eyes more
easily. However there are no anchor links to these, if these are necessary they can be added later.

Closes angular#3436
Closes angular#5762

src/ng/directive/ngShowHide.js

@@ -52,6 +52,11 @@
  *
  * Just remember to include the important flag so the CSS override will function.
  *
+ * <div class="alert alert-warning">
+ * **Note:** Here is a list of values that ngShow will consider as a falsy value (case insensitive):<br />
+ * "f" / "0" / "false" / "no" / "n" / "[]"
+ * </div>
+ * 
  * ## A note about animations with ngShow
  *
  * Animations in ngShow/ngHide work with the show and hide events that are triggered when the directive expression
@@ -200,6 +205,11 @@ var ngShowDirective = ['$animate', function($animate) {
  * </pre>
  *
  * Just remember to include the important flag so the CSS override will function.
+ * 
+ * <div class="alert alert-warning">
+ * **Note:** Here is a list of values that ngHide will consider as a falsy value (case insensitive):<br />
+ * "f" / "0" / "false" / "no" / "n" / "[]"
+ * </div>
  *
  * ## A note about animations with ngHide
  *